The thermal condensation of alpha amino acids to form polymers with loss of water has been known for many years. Early interest in such processes related to theories for formation of prebiotic polypeptides. For the purpose of testing such theories laboratory experiments used powdered L-aspartic acid, usually packed in the bottom of a flask which was then heated below the melting point of the acid. Such reactions were slow and took place over many hours. One such example is reported by Kokufuta et al. in Bulletin of the Chemical Society of Japan Vol. 51 (5) 1555-1556 (1978) "Temperature Effect on the Molecular Weight and the Optical Purity of Anhydropolyaspartic Acid Prepared by Thermal Polycondensation." The structure of anhydropolyaspartic acid has been thoroughly investigated such as by J. Kovacs et al. in J.O.C.S. Vol. 26 1084-1091 (1961).
In recent years many utilities have been suggested for anhydropolyamino acid. Such polyamides have been suggested as potential drug carriers by Neuse et al. in Die Angewandte Makronmolekulare Chemie 192 35-50 (1991) "Watersoluble polyamides as potential drug carriers." They have also been tested as scale inhibitors with respect to natural sea water and calcium sulfate in particular by Sarig et al. as reported by the National Council on Research and Development (NRCD 8-76, Seawater Desalination 150-157 (1977). Polyaspartic acid has been well known for its ability to disperse solid particles in detergent formulations, having been mentioned as a dispersant in numerous patents, a few of which are U.S. Pat. Nos. 4,363,497; 4,333,844; 4,407,722 and 4,428,749. As a departure from the usual manner of utilizing polyaspartic acid in detergent formulations it is reported in Australian Patent A-14775/92 that the polyamide is added to the wash liquor which, upon hydrolysis in situ, is converted into a biodegradable polypeptide builder. Also, as described in U.S. Pat. No. 4,971,724 to Kalota et al., it has been discovered that compositions comprising polyamino acids such as aspartic acid, when ionized at alkaline pH, effectively inhibit corrosion of ferrous metals in the presence of aqueous medium. Various derivatives of polyamino acids have also been made wherein attributes have been supplied by groups attached to reactive sites on the molecule. One such example is disclosed in U.S. Pat. No. 3,846,380 to Fujimoro et al.
Because of the various impending potential utilities of anhydropolyamino acids, interest in processes for preparing such compounds in large volume, particularly polyaspartic acid, has increased. This interest has resulted in several recent patents being issued which are directed to fluid bed systems; in particular, U.S. Pat. No. 5,219,986 to Cassata. Other such patents are U.S. Pat. Nos. 5,057,597 and 5,221,733 to Koskan and Koskan et al. respectively. In industrial processes for the production of large volumes of product it is highly advantageous to employ a continuous process for obvious reasons.
Polysuccinimide, the precurser to poyaspartic acid or this salts is prepared by means recently described in U.S. Pat. No. 5,057,597, U.S. Pat. No. 5,315,010 and U.S. Pat. No. 5,319,145. In all of these patents dryers of various types are employed in the reaction to convert aspartic acid into polyaspartic acid by thermal condensation. Catalysts such as phosphoric acid may be employed to aid in reducing the amount of time required to condense the L-aspartic acid while subjected to heat and the removal of water in known manner. However, reaction time of at least 2 hours is required in the most efficient means available. There is needed a more efficient means for providing polysuccinimide whereby a shorter residence time in the reactor is needed for large scale production of the polymer.